Cascading Style Sheets (CSS) is a coding language used to style HTML documents, or in other words, website pages. WordPress allows for additional CSS to be added to any site.
See the WordPress support page for more info.
CSS can be helpful when wanting to make small changes, such as changing font color or size, or larger changes, such as styling how an entire page looks.
To add CSS to your site, go to the Appearance section and then to Customize. The Additional CSS tab will be the last option at the bottom.
The page preview on the right side will reflect any CSS that is added to the left side. All CSS added will be applied to the entire site, so be cautious when applying CSS to general items that are located on multiple pages (such as heading tags).
To save changes, either click the Publish button at the top, or click the gear icon next to it for more options.
Here is what the Additional CSS section looks like along with example CSS used to customize this site.